Question

I would like to check to see if a box is checked using jquery.
My aim is to prevent the form from hiding if no option is selected.

$("#submit").click(function(){  
var opt = $('input[name=value]:checked').attr('value');
if(opt!='undefined'){
$.ajax({
type: "POST",
url: url,
data: 'option='+opt,
cache: false,
success: function(html)
{   
 alert(opt);

}
     });
}       
return false;
});

html

<form id="form" method="post" action="">
<input type="radio" name="value" value="1" />1<br>
<input type="radio" name="value" value="2" />2<br>
<input type="radio" name="value" value="3" />3<br>
<input type="radio" name="value" value="4" />3<br>
<input id="submit" type="submit" value="submit"/>
</form>

Answer 1

I suggest just checking $('input[name=value]:checked').length before doing anything else. ) If no element was checked, it will be equal to 0. In other words, replace

var opt = $('input[name=value]:checked').attr('value');

…with

var $checkedElement = $('input[name=value]:checked');
if (! $checkedElement.length) {
   // no option was selected, return false or alert
} 
var opt = $checkedElement.val();

Your original code, though, is pretty close, with just a single mistake: you should compare opt to undefined (a variable), not to 'undefined' (a string).

if (opt != undefined) { ... do the thing .. }

The opt != 'undefined' will actually always evaluate to false, as no radio button in your HTML has value 'undefined'. Again, consider the difference between false and 'false' – the latter is actually a truthy value (though not equal to true);